Set SRet flags properly in '-cxx-abi microsoft'.

Also,
- abstract out the indirect/in memory/in registers decisions into the CGCXXABI
- fix handling of empty struct arguments for '-cxx-abi microsoft'
- add/fix tests



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179681 91177308-0d34-0410-b5e6-96231b3b80d8